To investigate, in man, the systemic hematopoietic toxicity induced by regional radiotherapy during the management of neoplasiic disease. Special emphasis will be placed on identifying, in both the peripheral blood and bone marrow, the quantitative, morphologic, and functional correlates of this toxicity using presently available in vitro methodology for studying hematopoietic and stem cell activity. Studies will be made both acutely and during periods of hematopoietic recovery.